1. Field of the Invention
The present invention relates to honeycomb structural bodies capable of supporting catalyst therein to purify exhaust gas emitted from an internal combustion engine mounted to motor vehicles, etc.
2. Description of the Related Art
There have been known and used honeycomb structural bodies capable of supporting catalyst therein and purifying exhaust gas emitted from internal combustion engine mounted to motor vehicles, etc. For example, a honeycomb structural body is comprised of a plurality of cells and cell walls. Each of the cells is surrounded by the cell walls arranged in a lattice shape. Such a honeycomb structural body is arranged in the inside of an exhaust gas pipe through which exhaust gas is discharged. The exhaust gas has a high temperature because of being emitted from the internal combustion engine. When the exhaust gas having a high temperature is passing through the inside of the cells formed in the honeycomb structural body, the catalyst supported by the cell walls is activated by the exhaust gas having a high temperature. The activated catalyst in the honeycomb structural body purifies the exhaust gas. The purified exhaust gas is then discharged to the outside of the exhaust gas pipe.
Recently, because the vehicle emissions control of reducing motor vehicle emissions, etc. is becoming stricter year by year, there is a strong demand to more decrease toxic substances or harmful substances (cold start emission) generated immediately when an internal combustion engine starts. In order to achieve this demand, there have been proposed various methods of arranging a honeycomb structural body at a position more close to the internal combustion engine in order to speedily increase a temperature of the catalyst to an activation temperature of the catalyst. In other words, it is required for a honeycomb structural body to have a higher purifying performance
For example, a patent document, Japanese patent laid open publication No. JP 2008-18370 discloses a honeycomb structural body having a conventional structure in which an inner circumferential wall (hereinafter, referred to as the boundary partition wall) is formed between a central part and an outer circumferential part of the honeycomb structural body, and an opening ratio of the outer circumferential part is greater than that of the central part.
In general, such an opening ratio in an area is in inverse proportion to a cell density in the area. For example, the more the opening ration increases, the more the cell density decreases. Because the outer circumferential part has a resistance to flow exhaust gas, which is larger than that of the central part in the honeycomb structural body, the patent document JP 2008-18370 has proposed a structure in which the outer circumferential part has an increased opening ratio. This proposed structure makes an easy flow of the exhaust gas in the outer circumferential part.
This makes it possible to uniform the flow rate of the exhaust gas between the central part and the outer circumferential part, and speedily activate the catalyst supported by the entire honeycomb structural body. This can increase the purifying performance of the honeycomb structural body.
However, the honeycomb structural body disclosed in JP 2008-18370 does not have an adequately-required structural strength against various stress such as external stress generated when the honeycomb structural body is mounted to an exhaust gas pipe of an internal combustion engine. That is, although the boundary partition wall is formed between the central part and the outer circumferential part in order to keep an entire strength of the honeycomb structural body, it is not always possible for the boundary partition wall to have an adequate strength against the external stress.
In order to solve such a conventional problem, it can be considered to increase a thickness of the boundary partition wall and the cell walls around the boundary partition wall. However, even if a thickness of the boundary partition wall and a thickness of the cell walls around the boundary partition wall are simply increased, a feeding speed and a feeding amount of raw material are varied when the raw material is fed through an extruding die during a manufacturing process of a honeycomb structural body. As a result, there is a possibility of decreasing an overall strength of the honeycomb structural body manufactured.